This invention generally relates to macrocyclic polyesters. More particularly, the invention relates to a method for preparing macrocyclic oligoester compositions from intermediate molecular weight polyesters.
Macrocyclic oligoesters, also called macrocyclic polyester oligomer compositions, can be converted, often under isothermal conditions, to linear polyesters of high crystallinity and solvent resistance.
One method for preparing macrocyclic oligoesters is accomplished by the reaction of diols, such as ethylene glycol or 1,4-butanediol, with dicarboxylic acid chlorides, such as terephthaloyl chloride or isophthaloyl chloride, under specifically defined conditions. Other methods of preparing macrocyclic polyester oligomer compositions include the reaction of a dicarboxylic acid chloride, such as terephthaloyl chloride, with a bis(hydroxyalkyl) dicarboxylate, such as bis(4-hydroxybutyl) terephthalate.
Another macrocyclic oligoester preparation method is catalytic depolymerization of linear polyesters such as poly(4-butylene terephthalate) (xe2x80x9cPBTxe2x80x9d) and poly(ethylene terephthalate) (xe2x80x9cPETxe2x80x9d). Catalytic depolymerization macrocyclic oligoester preparation methods require that linear polyesters be purchased or manufactured prior to macrocyclic oligoester production. Producing macrocyclic oligoesters from high molecular weight linear polyesters necessitates handling of a high molecular weight material. The high molecular weight linear polyester material typically has a high viscosity, which requires costly equipment. In some instances it also requires many expensive finishing steps.
For example, prior art methods employ melt reactors that are capable of generating surface area in high viscosity high molecular weight material. Where poly(butylene terephtalate) is reacted with 1,4-butanediol, it is necessary to generate surface area to enable the diol to diffuse to the surface so that the reaction may proceed to build the molecular weight of the polymer. When high viscosity high molecular weight materials are prepared, diffusion of the diol from the polymer matrix is rate limiting and generating surface area by employing a melt reactor increases the reaction rate of the process of building the molecular weight of the polymer. Such melt reactors are highly engineered, energy intensive, and are run under relatively high vacuum conditions (e.g., 0.5 Torr) that are necessary to handle the high viscosity material.
Further, U.S. Pat. No. 4,590,259 to Kosky et al. describes a method for preparing poly(alkylene terephthalates) of blow molding grade in which a final step is conducted under conditions necessitated by the high melt viscosity of high molecular weight polyesters. A prepolymer preparation step is described in which, for example, a poly(butylene terephtalate) undergoes reaction with 1,4-butanediol. The resulting prepolymer undergoes reaction further with the diol in the presence of an inert gas to reduce the number of acid end groups, for example carboxylic acid end groups, to a desired level. Thereafter, solid state polymerization is conducted whereby the molecular weight of the polymer is increased under high vacuum conditions.
The invention generally relates to methods for depolymerizing linear polyesters to macrocyclic oligomer compositions. In one aspect of the invention, an intermediate molecular weight polyester is employed to prepare a composition comprising a macrocyclic oligoester. In one embodiment, a diol is contacted with a dicarboxylic acid or dicarboxylate in the presence of a catalyst to produce a composition comprising a hydroxyalkyl-terminated polyester oligomer. Thereafter, the hydroxyalkyl-terminated polyester oligomer is heated to produce a composition comprising an intermediate molecular weight polyester which preferably has a molecular weight between about 20,000 Daltons and about 70,000 Daltons. The intermediate molecular weight polyester is heated and a solvent is added prior to or during the heating process to produce a composition comprising a macrocyclic oligoester. The composition comprising macrocyclic oligoester includes, for example, a mixture of macrocyclic oligoesters and linear oligoesters. An optional step is to separate a macrocyclic oligoester from the composition comprising the macrocyclic oligoester.
Methods of the invention minimize operations and eliminate the need for costly equipment. Of the steps used to manufacture linear polyesters, only those steps necessary for ultimate recovery of macrocyclic oligomers are employed and integrated with the operations necessary to recover a macrocyclic oligomer composition. For example, the use of costly melt reactors is unnecessary. Later steps in prior art polyester preparation, in particular steps necessary to produce a high molecular weight product, are eliminated thereby providing a product adequate for conversion to macrocyclic oligomers without significant loss in yield.
Methods of the invention also permit the use of equipment that transports low viscosity materials, whereby these methods are less expensive than prior art alternatives. According to the invention, low viscosity intermediate molecular weight polyesters are employed to make macrocyclic oligoesters. However, compared to the above-described methods employing high molecular weight polymers, methods of the present invention employing intermediate molecular weight polyesters have an increased number of linear polyesters with uncyclizable end groups remaining after cyclization. To promote further reaction to form macrocyclic oligoesters from intermediate molecular weight polyesters, it may be desirable to remove an uncyclizable end group, e.g., a hydroxyalkoxy group, from the linear polyester to create an intermediate molecular weight polyester capable of forming a macrocyclic oligoester. Removal of an end group results in by-product diol, which may be the starting material diol, i.e., the first compound.
In one embodiment, one or more solvents are employed for removal of by-product diol(s) by distillation, as the by-product diol(s) should be removed before cyclization takes place. The distillation may be, for example, azeotropic distillation, or distills or co-distills of the by-product diols may distill off at a temperature below the boiling point of the solvent. Thus, energy- and equipment-intensive methods, for example, melt reaction, necessitated by the high melt viscosity of the product are replaced by a simple distillation. In addition, the use of solvent reduces the viscosity of the macrocyclic polyester product, which enables the process to employ less costly equipment that is capable of transporting a low viscosity material.
Thereafter, the removed end groups, which typically result in by-product diols, may be transported and purged as waste from the process. Alternatively, a recycling step may be employed to reuse the by-product diol as a reactant in the process. Likewise, the volume demand on an existing recycling step may increase in order to transport the by-product diol created during the process. Despite the addition of such a recycling step or the additional volume demand on an existing recycling step, methods of making macrocyclic oligoesters from low viscosity intermediate molecular weight polyesters can be more cost-effective than prior methods. The cost benefit is due in part to the ability to employ simple distillation in place of expensive finishing steps. Also, transporting low viscosity polyesters is less costly then transporting high viscosity material. Macrocyclic oligoester production methods according to the invention can be conducted continuously, semi-continuously, according to a batch methodology, or a combination thereof.
In another aspect, an embodiment of a method for preparing a macrocyclic oligoester composition includes providing an intermediate molecular weight polyester, a solvent, and a catalyst. The catalyst may comprise, for example, a tin compound or a titanate compound. The intermediate molecular weight polyester, solvent, and a catalyst are heated to produce a composition comprising a macrocyclic oligoester. The method may further include separating the macrocyclic oligoester from the composition comprising the macrocyclic oligoester.